The present invention relates to heat exchangers and more particularly to heat exchangers for refrigeration systems. Specifically, the present invention relates to a fluid temperature measuring device for use with a heat exchanger of the type wherein a fluid to be heated or cooled is passed through heat transfer tubes which empty into a channel.
In a heat exchanger of the flooded evaporator type, having water which is cooled by flowing through heat transfer tubes which are immersed in refrigerant, it is desirable to monitor the temperature of the water in the tubes because freezing water can obstruct or possibly rupture a tube. The water may freeze due to refrigerant undercharging of the heat exchanger or during low load operating conditions. When undercharged more heat transfer occurs from the lower tubes in the heat exchanger than from the higher tubes because the lower tubes are covered with liquid refrigerant while the higher tubes may be only partially covered with liquid refrigerant. Therefore, the water in the lower tubes may be cooled to a temperature below their designed temperature operating range if the average water temperature in the heat exchanger is maintained at the design level. Also, during low load operating conditions the refrigerant level in the heat exchanger decreases, in part due to a decrease in the boiling action in the heat exchanger whereby some of the higher tubes in the heat exchanger are not covered by liquid refrigerant. Therefore, as occurs in the undercharged situation, more heat transfer takes place from the lower tubes in the heat exchanger and the danger of freezeup in these tubes is present.
A conventional temperature probe used to monitor the water temperature in a heat exchanger of the above-described type comprises a bulb containing an expandable fluid, such as a refrigerant, with a capillary tube connected to the bulb. The entire bulb of the probe is placed inside one of the heat transfer tubes which empty into a channel. The bulb may be held in place by a flexible metal clip or by forming the bulb in a resilient bow-shape so that the bulb must be flexed when inserted into the tube whereby the bulb holds itself in place in the tube after being inserted. The capillary tube of the probe is inserted through a wall of the channel and connects the bulb to an electronic control system located outside the channel. The temperature probe generates a pressure signal in response to the temperature of the fluid surrounding the bulb of the probe. A pressure transducer may change the pressure signal into a corresponding electrical signal for processing by the electronic control system, or the probe may be connected directly to a pressure switch.
Normally, the bulb of the temperature probe is inserted into one of the lower heat transfer tubes in the heat exchanger since the lower tubes are more likely to encounter freezeup problems for the reasons discussed previously. Also, the bulb of the probe is designed to provide frictional resistance to the flow of water through the tube in which it is inserted. This insures that the probe measures the lowest water temperature in the heat exchanger since restricting the water flow through the end of the heat transfer tube substantially reduces the water flow rate through this tube relative to the rate of water flow through the other tubes of the heat exchanger. Therefore, the water flowing through the tube having the bulb is cooled slightly more than the water flowing through the other tubes. If the probe senses a water temperature indicating a possible freezeup problem then the electronic control system responds to shut down the heat exchange system or to take other such appropriate steps.
One disadvantage of the conventional temperature probe is that to replace the bulb of the probe the channel of the heat exchanger must be drained and then opened to reach the bulb. Also, it is necessary to use a pressure transducer to interface the conventional temperature probe with an electronic control system if an action other than simple shutdown is to be controlled. Such a pressure transducer is costly and can present reliability problems.
Other kinds of temperature probes, other than the pressure type just described, are known for measuring the temperature of a fluid. However, the use of these probes with a heat exchanger for the present purposes is not easily accomplished. Usually, it is necessary to place a temperature sensing element of these probes directly into a tube rather than locate it in the channel. One reason for locating this element inside a tube is that there is a greater change in fluid temperature in an individual tube in the lower tubes of the heat exchanger than in the fluid temperature in the channel for a given freezeup condition or other such problem. The fluid in the channel is a mixture of the fluid from all the tubes and therefore may undergo only a small temperature change while the fluid temperature within the individual tube undergoes a larger change. Also, typically, the fluid temperature in the channel is controlled to be maintained at a constant level while the fluid temperature in the individual tubes is varying due to the operating conditions in the heat exchanger. The necessity for placing these other temperature probes inside a single tube presents the same placement and replacement problems as discussed with respect to the conventional pressure type temperature probe.